On Thu, 14 Jan 2021 03:46:08 +0000
The Natural Philosopher  wrote:

>On 13/01/2021 22:04, Joe wrote:
>
>>
>> Lithium cells have a linear-ish discharge curve, from 4.2V down to
>> around 3V.
>
>They do.
>
>> Any sensibly designed lithium battery will cut off its
>> output at the chosen lower bound,
>
>No, it wont. That isn't in the *battery* - that's in whatever is drawing
>current off it. I have many lthium packs with no such ptotection
>
>
>
>> because if it completely discharges,
>> it's dead forever. Not a 'steep' decline, a 'fall off the wall' decline.
>
>No, again it goes on down to zero, 3V is just approximately the point at
>which irreversible damage starts to happen.
>
>>
>> It's a good idea to have some independent means of anticipating this
>> point.
>>
>The way to run a lithium in this application is to use a 2 cell lithium,
>a constant voltage constant current charger limited to 8.4V and probably
>no more than the battery mAh capacity divided by one hour... and a
>switched mode 5V regulator to feed the Pi and then somehow monitor raw
>battery voltage and switch it all off when it gets to say 6.6V and hope
>that the SMPS and monitoring circuit don't then still draw enough to
>damage the battery, or even if it is take it on the chin and replace the
>battery when that happens.

To get round that use a latching relay 'start' circuit, and when the
battery voltage gets too low unlatch the relay-  Zero power demand then.


--
W J G

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